ITU-R BS.1770 Loudness and LUFS Explained

BS.1770, in one sentence

ITU-R BS.1770 is the international standard that specifies how to measure the perceived loudness of audio — it defines the frequency weighting and the gating rules that turn a waveform into a single LUFS number. Spotify measures its loudness normalisation according to ITU 1770, and the EBU R128 broadcast recommendation is built on BS.1770.

K-weighting: measuring loudness the way ears hear it

Human hearing isn't equally sensitive to all frequencies, so BS.1770 first passes the audio through a K-weighting filter with two stages: a high-shelf pre-filter that gently boosts high frequencies (roughly +4 dB above ~1.5 kHz, modelling the head-related boost), and a RLB high-pass that rolls off very low frequencies (around 38 Hz) because deep bass contributes little to perceived loudness. The result correlates with how loud a track actually sounds, not just its raw energy.

Gating: ignoring the silences

A song has quiet passages that would drag the average down, so BS.1770 applies two gates. The absolute gate discards blocks below -70 LUFS (near-silence). The relative gate then discards blocks more than 10 LU below the average of what remains. What's left is measured to produce the Integrated LUFS — the headline loudness figure platforms use.

How a LUFS reading is built

The pipeline is: 1) K-weight both channels; 2) split into 400 ms blocks with 75% overlap; 3) compute mean-square power per block; 4) apply the -70 LUFS absolute gate and the -10 LU relative gate; 5) average the surviving blocks and convert to LUFS. Loudness Range (LRA) is derived separately from the spread of 3-second short-term measurements.

BS.1770 vs EBU R128

BS.1770 defines the measurement method. EBU R128 is a broadcast recommendation built on BS.1770 that adds a target (-23 LUFS), a true-peak ceiling (-1 dBTP) and the LRA descriptor. Streaming platforms use their own targets (around -14 LUFS) but the same BS.1770 measurement underneath. So a BS.1770-accurate meter reads correctly for both broadcast and streaming.

Why an exact implementation matters

Many web meters approximate K-weighting with generic parametric-EQ (RBJ) biquads, which drift from the spec's exact filter — the deviation can reach several tenths of a LU in the presence region around 3 kHz. Quantara Music measures to the BS.1770-4 standard, so the reading matches it at both 44.1 kHz and 48 kHz. Accurate metering is the whole point: if the number is wrong, every loudness decision built on it is wrong.

Frequently asked questions

What is K-weighting?

K-weighting is the frequency filter in BS.1770 that shapes audio to match human loudness perception before measurement: a high-frequency shelf boost plus a low-frequency roll-off. It's why LUFS tracks perceived loudness rather than raw energy.

What's the difference between BS.1770 and EBU R128?

BS.1770 is the ITU measurement method (K-weighting + gating). EBU R128 is a broadcast recommendation built on it, adding a -23 LUFS target, a -1 dBTP ceiling and Loudness Range. Streaming uses BS.1770 with different targets (~-14 LUFS).

Why do different LUFS meters give different readings?

Because many meters approximate the K-weighting filter or handle gating and sample rates differently. A spec-exact meter (the standard's own filter and gating at every sample rate) removes those discrepancies.

Is Quantara's loudness meter spec-accurate?

Yes. Quantara measures loudness to the ITU-R BS.1770-4 standard, including its gating, so integrated LUFS matches the standard at 44.1 kHz and 48 kHz.